Format strings: a character literal should be printed with %c, not %d.

The type of a character literal is 'int' in C, but if the user writes a
character /as/ a literal, we should assume they meant it to be a
character and not a numeric value, and thus offer %c as a correction
rather than %d.

There's a special case for multi-character literals (like 'MooV'), which
have implementation-defined value and usually cannot be printed with %c.
These still use %d as the suggestion.

In C++, the type of a character literal is 'char', and so this problem
doesn't exist.

<rdar://problem/12282316>

llvm-svn: 169398